Improved hyperthermia heating systems that have flexibility in their deposition patterns are becoming available commercially, and even more flexible systems are being developed in research laboratories. To best use these devices one must determine how to deposit the power optimally, for which one needs a clear thermal treatment goal including an ideal temperature distribution. This paper presents definitions for these quantities, and theoretically investigates the relative effects of thermal conduction and blood convection cooling on them. It is shown that both cooling mechanisms can be important clinically, and that the presence of 'large' blood vessels can have a major impact on the optimal power deposition patterns. In particular, an ideal heating system would deposit enough power both outside the treatment volume in order to preheat the incoming blood to the desired treatment temperature, and at the treatment volume boundary in order to overcome thermal conduction effects. The degree to which any heating system can approach the ideal temperature distribution is shown to be strongly influenced by the size and shape of its power deposition field, i.e. its conformability, or ability to conform to the boundary of the desired treatment volume.